Fundamental photonics question

Hi all,

I have a (silly) question. How terminations are achieved in photonic waveguides ? Is it just like RF where you need to the waveguide impedance and characteristic impedance of the termination equal ?

How do terminations look like in case of photonics ?

How is energy absorbed ? Is light allowed to just scatter out or is there any other mechanism ?

Looking forward to some answers to this rather basic question.

Regards
svarun

optical engineering is quite similar to microwave engineering. In microwaves, when you go from one transmission line impedance to another, there is a reflection. You can reduce the microwave reflection by impedance matching, such as a quarter wave transformer. A quarter wave transformer has an impedance of √{Z1*Z2). Microwave engineers design networks with S parameters. Microwave guys make reflection oscillators by using a tank circuit and an active negative resistance device.

In optics you travel in one "index of refraction" η media, and if you hit a different η media you have a reflection. If you use a quarter wave coating, such as on a lens surface, of η = √(η1*η2), then you have no reflections. The only obvious difference is that the "quarter wave thick coating" is pretty physically thin, due to the small wavelengths. Optical engineers design lens networks with Transmision matrixes. Optical engineers make oscillators (lasers) with two reflectors and a travelling wave media that acts a lot like a negative resistance.

Etc, etc.

About the only thing the optics guys use a lot that the microwave guys never do, is the concept of critical angle/total internal reflection. This is used in fiberoptics to keep the energy confined in the center of the fiber by coating the center with another glass with a slightly different η. Optics guys use a lot of intererometry stuff too, that the microwave guys do not find useful unless they are above 40 GHz.

Hi biff44,

Thanks a lot for your reply. If you are familiar with the Beam Propagation Method for analyzing photonic components, I have a question. BPM assumes 'z' direction propagation,. What happens if you need to analyze bends ? Do you do co-ordinate transformations ?

svarun